This invention relates to drilling boreholes in the earth, and more particularly to preventing buckling of the drill string.
The problems encountered in drilling through the earth to very deep depths have been well documented and successfully solved. These problems are exacerbated in so-called "extended reach drilling" where the path of the drill bit deliberately deviates substantially from the vertical direction. The insertion of tubulars, drill strings, casings, and tubing into very high angle boreholes is particularly difficult.
Recently, sophisticated technology of electronic measuring and data transmission has been applied to this problem. Many state of the art systems accurately track and control the path of the drill string through the subsurface formations. For example, U.S. Pat. Nos. 3,622,971-Arps and 4,021,774-Asmundson, describe apparatus for tracking the path of a drill string through the earth from measurements of azimuth and inclination. The Arps patent includes a computer at the surface of the earth for determining the path from the down hole measurements.
U.S. Pat. No. 3,968,473 shows apparatus for measuring the weight on the drill bit and the torque applied to the drill string. U.S. Pat. No. 3,759,489 describes apparatus for automatically controlling the weight on the bit.
One problem which has not been adequately addressed is the buckling of segments of the drill string. This causes deflections which in turn cause forces against the hole wall which increase the frictional drag. Also, buckling stresses cause pipe fatigue.
Some sections of the boreholes may have inclinations 80.degree. to 90.degree. (or greater) from the vertical in which the pipe within that section will not slide through the hole with just the force from its own weight. In this situation, sections of the pipe have to be pushed in order to move.
As a pipe is pushed through a hole, it will flex and buckle. At each contact to the wall, an additional force will be applied against the wall of the borehole causing additional drag. This creates the cumulative situation of added drag causing needed additional axial force which causes more buckling, more force against the wall and more drag, a snowballing effect. A point will be reached where, for a given set of conditions, the force to push the pipe is not available or the pipe can fail. Many alternatives exist to change the given conditions, such as: changing the tubular strings; changing the borehole configuration, i.e., casing, or hole sizes; changing the coefficients of friction; and devising means to create a pushing force.
The criteria for buckling in a drill string are known and are described in: Lubinski, Arthur, and Woods, H. B., "Factors Affecting the Angle of Inclination and Dog-Legging in Rotary Bore Holes," API Drilling and Production Practice, 1953, pp. 222-250; and Woods, H. B., and Lubinski, Arthur, "Practical Charts for Solving Problems on Hole Deviation," API Drilling and Production Practice, 1954, pp. 56-71. The application of these criteria to indicate buckling in actual drilling situations, and in the simulation of such drilling, is an object of the present invention.
It is an object of the present invention to determine whether or not buckling of the drill string will occur under certain drilling conditions so that these conditions can be modified or avoided.